Unusual PEComa With PRCC::TFE3 Fusion Mimicking Sinonasal Tract Melanoma

Background: We report a nasal cavity unusual perivascular epithelioid cell tumor (PEComa) mimicking mucosal melanoma. Methods: Immunohistochemistry was performed using BenchMark Ultra and panel of antibodies. The Ion Torrent platform and Ion AmpliSeq cancer hotspot panel were utilized for DNA genotyping. Target-specific RNA libraries for the detection of fusion transcripts were constructed using Archer Universal RNA Reagent Kit v2 and Archer FusionPlex Solid Tumor panel and sequenced on the MiSeqDx instrument. Results: The tumor, diagnosed in 46-year-old female, was composed of spindle cells, and lacked pigmentation. Immunohistochemically, it showed a patchy HMB-45 positivity. Other melanocytic markers (S100 protein, Melan-A, SOX10) were negative. The tumor cells were weakly positive for KIT (CD117) while negative for smooth muscle actin, pancytokeratin cocktail (AE1/AE3), and synaptophysin. Diagnosis of primary sinonasal tract mucosal melanoma was favored. Additional molecular studies detected PRCC :: TFE3 fusion as the sole genetic change, and suggested the diagnosis of unusual PEComa. Previously, TFE3 fusions were reported in a subset of PEComas but not in melanomas, while PRCC involvement has only been documented once in an ocular PEComa. Immunohistochemistry revealed strong nuclear TFE3 expression concordant with the molecular findings. Conclusions: This report emphasis the importance of molecular testing in the differential diagnosis between PEComa and melanoma, especially when the tumor arises in a site typical of melanoma but showing an unusual morphology and immunophenotype. The detection of TFE3 fusion transcripts suggested the diagnosis of SNT PEComa, although it cannot be excluded that this and similar tumors represent a distinct diagnostic category.